12ax7 cathode grounded

[sbirkenstock]

Hi again,

I was tinkering around and put a 10k pot as the cathode resistor for V1.
I wanted to figure out the perfect grid resistance.
(Always clean Strat into a BF Vibroverb Style 6L6 amp with no reverb or tremolo)
Surprisingly the amp still worked pretty well when I set the pot to 0 (= cathode grounded).
How could that be? I could hardly measure any bias at the grid.
Why is that working? And not bad, it does sound pretty good and not scratchy or anything.

That reminded me of very old tweeds that grounded the grid with a 5 meg resistor, where a bias develops due to grid current.
Then I figured out (I started another post on this) that the standard 1 Meg in the Fender input actually get´s "bypassed" by the guitar resistance, in my case 5.7K).
That explains why there was hardly any bias to measure.

So I coupled the guitar with a .1mf cap.
I tried that with a 5meg resistor to grid, cathode grounded, and the grid develops ca. 0,3V as bias.
Then I tried a 1 Meg grid to ground, and that sounded better to me.
It results in 0,2V bias on the V1 grid.
It sounds very open and good.
Usually the bias (grid to cathode voltage) should be 1,5V to 2V.
I am really surprised by the rsults, as the old 5meg grid to ground Fender (does anybody know a model or the name for this?)
are supposed not to sound any good.
Seems to me it´s not caused by this part...


Best regards,

Stephan

[martin manning]

Some people like the sound of grid-leak bias. With the blocking capacitor between the guitar and the grid, the bias doesn't change much using 1M to several or even 10M grid leak resistors, and it will be pretty close to 0V Vg-k. The difference in sound is due to the much higher input impedance seen by the pickup. Most data sheets don't show any curves for bias voltages greater than zero, but they exist, and you can get an unclipped output signal with the grid biased at 0V.

[Andy Le Blanc]

zero bias.... the bias is not there until there is signal.
the max gain charts are notated at 5% harmonic distortion.
they'er ok as a first stage small signal gain stage, but get tricky with multiple stages.

you can build a simple rig with them, a basic pre, volume tone... and a paraphrase invertor... nice tone... grind ete, but they run out of bias when you really push them and can cut out, still a nice rig if you don't push it in to extremes